The term transgender is more common[1] and is preferred by many transgender people; several dictionaries, style guides and other authorities specifically proscribe transgendered, often comparing its use to the hypothetical use of blacked in place of black or lesbianed in place of lesbian.[2][3][4][5][6]

^GLAAD Media Reference Guide - Transgender Issues: Problematic: "transgendered". Preferred: transgender. The adjective transgender should never have an extraneous "-ed" tacked onto the end. An "-ed" suffix adds unnecessary length to the word and can cause tense confusion and grammatical errors. It also brings transgender into alignment with lesbian, gay, and bisexual. You would not say that Elton John is "gayed" or Ellen DeGeneres is "lesbianed," therefore you would not say Chaz Bono is "transgendered."